This invention relates to a method of treating waste water by means of a fluidized adsorption bed. Pollution by waste water has become one of great social problems. Pollutants of waste water includes miscible and immiscible forms of pollutant. Suspended solids included in immiscible pollutants of waste water can be effectively removed mainly by filtration during treatment of waste water. Further, immiscible oily matter included in the immiscible pollutants can also be effectively treated by combination of coagulation and flocculation. However, filtration coagulation or flocculation exerts little advantageous effect on treatment of miscible pollutants. One of the miscible pollutant-treating methods is to remove miscible pollutants by transforming them into the immiscible form, using activated sludge. Another miscible pollutant-treating method is to adsorb miscible pollutants by adsorbent. Adsorption by activated carbon has proved very effective for removal of particularly nonionic miscible pollutants. Since treatment of waste water is often primarily intended to eliminate nonionic miscible pollutants, application of activated carbon occupies a very important position in this particular field.
The oldest waste water-treating method based on activated carbon comprises the steps of throwing powdered activated carbon into waste water to remove pollutants by adsorption and filtrating out pollutant-laden powdered activated carbon to clean waste water. Even at present, this process is still locally followed. However, pollutant-laden powdered carbon whose regeneration is accompanied with considerable difficulties is as a rule simply dumped. Since such dumped material raises a problem of secondary pollution, the present trends is toward application of granular activated carbon which enables said spent carbon to be regenerated instead of being thrown away and used again by recycling.
A known waste water-treating method based on granular activated carbon is a fixed bed of granular activated carbon. A process proposed by Dostal, K. A., et al (Journal of AWWA, Sept. 1966, page 1170) has the advantage that since a mass transfer zone is formed in a fixed bed of granular activated carbon, a relatively long period time is required for a break-through point (that is, a maximum acceptable effluent concentration used in the above-mentioned literature) to be reached. However, this proposed waste water-treating process has the drawbacks that granular activated carbon of a small particle size, although such an activated carbon can attain quick adsorption when applied to the process, leads to unfavorably large pressure loss, thus only allowing application of granular activated carbon having a relatively large particle size and consequently performing slow adsorption; the fixed bed of granular activated carbon is clogged by immiscible pollutants entrained with waste water; pressure loss tends to be increased due to the fixed bed being closed by the growth of, for example, microbes, algae and mosses on the surface of packed activated carbon; a complicated operation such as back washing is required to clean the fixed bed; and waste water tends to be biased due to channeling.
An attempt to resolve the drawbacks accompanying the above-mentioned waste water-treating method based on a fixed bed of granular activated carbon, has been made using a fluidized bed in an ordinary sense (disclosed, for example, in Journal of Water Pollution Control Federation, by Weber, W. J. Jr., et al, April, 1965, 37 (4), page 425). This fluidized bed process has advantages over the fixed bed process in that a bed of activated carbon is not clogged up with immiscible substances or microbial suspensions; low pressure loss admits of application of powdered activated carbon having a small particle size and consequently carrying out quick adsorption.
Still, this proposed fluidized bed process has the drawbacks that since back mixing of granules of activated carbon takes place due to fluidization (FIG. 1), the mass transfer zone described in connection with a fixed bed of activated carbon tends to extend upward in the direction in which granules of activated carbon are fluidized, causing the break-through point of activated carbon to be soon reached, and consequently making it necessary frequently to replace spent activated carbon.
An attempt (Japanese patent disclosure No. 79169, 1973) was proposed to eliminate the drawbacks of the conventional fluidized bed type waste water-treating process based on granular activated carbon in which the above-mentioned back mixing of granules of activated carbon occurred. According to this attempt, a multi-staged fluidized bed was constructed by partitioning the adsorption section of a fluidized bed by a large number of porous plates to prevent the back mixing of granules of activated carbon, thereby forcefully providing a mass transfer zone apparently approximating that which was provided in the fixed bed. However, this multi-staged fluidized bed was accompanied with the drawbacks that the apparatus had a complicated and bulky construction; merit could not be expected, unless the apparatus was coupled to a regeneration device to regenerate the adsorptive power of granular activated carbon by continuous circulation; and the apparatus did not offer economic advantages except when the apparatus was applied on a tremendously large scale.